1. Technical Field
The invention relates to a zoom lens having a zoom ratio of about 3.3 for use in a video camera, an electronic still camera or the like, and more particularly to a small-sized zoom lens that is used on a relatively telephoto side and suited for a dome monitoring camera or the like.
2. Related Art
There is known, as a zoom lens for use in a vide camera or a digital still camera, a zoom lens having a four group system in which the power of the zoom lens varies by moving a second group along the optical axis with first and third groups fixed and a position of an image plane is corrected by moving a fourth group. For example, JP 2006-3589 A (corresponding to US 2005/280901 A) and Japanese Patent No. 3391342 (corresponding to U.S. Pat. No. 6,404,561) have proposed the following zoom lens. An optical system includes a first group having a positive refractive power, a second group having a negative refractive power, a third group having a positive refractive power and a fourth group following the third group. The first group is formed of, in order from the object side, three lenses, that is, a negative lens having a convex surface directed to the object side, a positive lens having a convex surface directed to the object side and a positive lens having a convex surface directed to the object side, to thereby ensure high magnification ratio from a wide end to a telephoto end.
Recently, as to the optical system for the monitoring camera, demand for a telephoto side is rising due to its installation site. Also, because of spread of a small dome camera, demand for miniaturization of the optical system also becomes severe. Under such a circumstance, the zoom lenses disclosed in JP 2006-3589 A (corresponding to US 2005/280901 A) and Japanese Patent No. 3391342 (corresponding to U.S. Pat. No. 6,404,561) can be reduced in the total length of the lens by reducing the zoom ratio. However, it is difficult that such an optical system required for telephoto use is reduced in size so as to be received in the dome camera even if the zoom ratio is made small. Consequently, there is a desire for a small-sized zoom lens suited particularly for a dome monitoring camera or the like.